In a cold rolling mill, hot rolled steel strips are rolled at room temperature to achieve improved surface quality and mechanical properties of the final cold rolled products. However, extensive deformation of the steel at room temperature during the cold rolling operation significantly reduces the ductility of the cold rolled sheets. In order to render the cold rolled sheets amenable for subsequent operations, e.g. deep drawing of auto body parts, the cold rolled steel coils need to be annealed.
During the annealing operation, deformed microstructures of the cold rolled sheets are stress relieved, and accordingly recovery, recrystallisation, and grain growth take place.
Thus, the cold Rolled steel coils need to be annealed to obtain desired metallurgical properties in terms of strength and ductility levels. To achieve this, this cold rolled steel coils are stacked one above other and placed in a heating chamber. The heating chamber heats the coils to temperatures of 400-500° C. The heating process is followed by a cooling cycle. The cooling cycle uses hydrogen to take the heat away indirectly by cooling a hood of the furnace. Efficiency of the cooling cycle depends on the rate at which heat can be extracted from the hydrogen within the confinements of the system.
Batch annealing furnace typically comprise a base unit provided with a recirculation fan and cooling means. On the base unit, several cold rolled steel coils are placed one above the other, separated by a plurality of circular convector plates. These cylindrical shaped coils with outer diameter (OD) in the range of 1.5-2.5 m, inner diameter (ID) 0.5-0.7 m, and widths of 1.0-1.4 m, weigh around 15-30 t each. These are the typical data, which widely vary from plant to plant depending upon the overall material design. After loading the base with the coils, a protective, gas tight cylindrical cover is put in place and hydrogen gas is circulated within this enclosure. A cylindrical hood for the gas or oil fired furnace hood is placed over this enclosure. The protective cover is externally heated through radiative and convective modes of heat transfer, which heats the circulating hydrogen gas. The outer and inner surfaces of the coils get heated by convection from the circulating hydrogen gas and by radiation between the cover and the coil. The inner portions of the coils are heated by conduction.
During the cooling cycle, the furnace hood is replaced with a cooling hood and the circulating gas is cooled.
There are generally three known strategies that are followed in batch annealing furnace, namely:                (a) AIR/JET cooling in which compressed air hits the cooling hood at high pressures.        (b) SPRAY cooling in which water is sprayed directly onto the cooling hood.        (c) BY-PASS cooling in cooling in which a gas flowing in the inner cover is tapped and cooled, using a heat exchanger. The efficiency of the heat exchanger determines the rate of cooling of the gas.        
Commonly used mechanism for increasing the heat transfer rate, are:                (a) Increasing the number of tubes and corrugations per tube inside the heat exchanger.        (b) Using water at a lower temperature obtained from a chilled water line.        
Both methods (a) and (b) are costly and hence do no find acceptance under the present circumstances.